Electromagnetic shields are generally constructed of a metallic material and are positioned around radiating components to constrain electromagnetic fields produced by the radiating components. By shielding the radiating components, which may be microstrip transmission lines or surface mount electrical components, the electromagnetic energy emitting from them will not cause undesired effects in neighboring circuitry or equipment. Alternatively, electromagnetic shields may be positioned around components which are sensitive to electromagnetic energy to minimize their exposure to unwanted electromagnetic fields. Due to the malleability of metallic materials, the shields may have almost any geometric shape which allows for convenient electromagnetic isolation of radiative circuitry or equipment from susceptible circuitry or equipment.
The metallic materials typically used for an electromagnetic shield are copper or stainless steel. These metals are selected primarily due to their inherently high values of conductivity which effectively restricts the flow of radiated electromagnetic energy. Although metallic electromagnetic shields typically provide effective electromagnetic shielding, they are limited by the effectiveness of the peripheral seal which seals the shield around the affected component.
Attachment of metallic shields to various substrate materials has certain limitations. Typically, metallic shields, which are incorporated on printed circuit boards, require small apertures located about their peripheral to allow reliable attachment of the shields to the printed circuit boards. Depending on the number and separation of the apertures, the metallic shields leak electromagnetic energy from the spaces between the apertures. Additionally, metallic shields typically have thermal coefficients of expansion in the range of 14 ppm/.degree. C. to 24 ppm/.degree. C. which limits the types of substrate materials the metallic shields can be soldered to and still maintain solder joint integrity over deviations in ambient temperatures. Further, attachment of metallic shields to various substrate materials is generally an unreliable and tedious procedure due to the added parts associated with the shield and due to the necessity that the shield's cover be attached after the substrate is solder reflowed. Attachment of the shield's cover prior to solder reflow is prohibited because solder joint integrity of the enclosed circuits could be compromised.
Recent technological advances have allowed electromagnetic shields to be fabricated from a printed circuit board. For a detailed discussion of fabricating shields from printed circuit boards refer to pending U.S. Patent application No. 07/806382, entitled "RF Shield Fabrication Method", assigned to Motorola Inc. While this technology provides many advantages, it does not address the technological concern of having different materials for the substrate and the electromagnetic shield.
Therefore, a need exists for an electromagnetic shielding assembly that allows complete enclosure of an electrical circuit on a substrate of a different material than an electromagnetic shield and that reduces the fabrication complexity of an assembly which includes the shield and the circuit.